1. Technical Field
The present invention relates generally to a device that can provide a side load capability which can be added or designed into any linear actuator/guide system, within the original physical envelope, to prevent undesired loading and excessive friction from being transmitted to the axial drive mechanism of the actuator system.
2. Background Art
A common linear actuator comprises a motive force--e.g., manual, electrical, pneumatic, hydraulic, etc.--driving a driver--e.g., ball screw and ball nut, pneumatic cylinder, hydraulic cylinder, etc.--which in turn positions an actuating member or piston. A conventional electromechanical linear actuator, comprising a motor driving a screw and nut assembly, is described in U.S. Pat. No. 4,500,805 to Swanson. A rotary mechanical motion transmission means is operated in unison with the motor. An elongated housing has one end in communication with the motion transmission means and an opposite end portion remote therefrom. An actuator assembly telescopically received within the housing at its opposite end portion is movable linearly relative thereto in one and an opposite direction for projection from the opposite end portion of the housing in varying degree.
Presently, linear actuators can be adapted for an almost unlimited number of applications, including pick and place operations involving substantial weight, part transfer, tool positioning, and lifting. In normal operation, the actuator assembly can be provided with a load and thereafter be extended and retracted. In such operation, the assembly may be subjected to bending moments in the form of forces cantilevered from the actuator mounting plate, forces "off-center" in the plane normal to axial motion, forces in planes that are parallel and/or offset to the axis of the actuator, and combinations thereof. Such bending moments lead to binding and/or side loading which causes wear and deformation of parts, and may lead to accelerated malfunction of the actuator.
Moreover, the bending moments to which an actuator is subjected vary relative to the extension position of the actuating or telescoping member, further contributing to the wear of the actuator. The side load acting upon the actuating member increases exponentially as the member is extended. Additionally, the side load acting upon the actuating member magnifies as the load magnitude increases.
Accordingly, linear actuators must be adapted to handle side load and corresponding bending moments, as well as withstand high pressure-velocities created by the movement of the actuating member. Traditionally, attempts have been made to isolate linear actuators, along with their main driver mechanisms, from bending moments. One solution to compensate for bending moments, is to provide a linear actuator with an external counterbalance/counterweight system or, alternatively, to provide an external auxiliary guide and support system. However, such external systems for actuators have not been wholly satisfactory because they significantly increase the size of the overall envelope by adding external components to an existing actuator.
Additionally, traditional counterbalances and counterweights that provide either a constant or a linear progressive force create excessive loading and wear in situations where the actual bending moments are less than the maximum bending moments expected. Thus, an external side load compensation device may subject a linear actuator to binding even if the load is removed and the actuating member is in a relatively retracted position.
The absence of a counterbalance system limits the length of the actuating member, and therefore the extent to which the member may be extended in use. This is because binding or excessive friction will occur in a plain or journal bearing design whenever the moment arm length exceeds twice the distance between the axial support bearings of the actuating member, regardless of the load magnitude. The work envelope must therefore be expanded in order to accommodate external guide systems which can increase side load capacity as well as extension length. However, increasing the size of the external guide system to accommodate actuating member extension also increases the load for the motor to handle, thus again accelerating wear and possibly malfunction of the motor.
It is the general object of the present invention to provide a side load compensation mechanism which avoids the drawbacks of the known art while enabling the linear actuator to perform its traditional axial tension and compression (i.e., extension and retraction) functions without encumbrance from simultaneous bending loads.
A further object of the present invention is to provide a side load compensated linear actuator which applies and resists axial forces and supports substantial bending moments without expanding the conventional linear actuator physical envelope.
Still another object of the present invention is to provide a unique, fixed or adjustable internal, pre-loaded, nonlinear cam mechanism which may be incorporated in any linear actuator regardless of motive power.